Shift camera focus on the same path
Actually there is very easy way. Give the camera, one "Track to" constraint for each planet, that You want the camera focus on, and then Animate Influence of the constraints.
Although if You want different planets to be in camera focus in Depth of field settings, You need to create an empty, make it focus target and "Track to" target and give it "Copy Location" constraint for every planet. Then Animate influence of "Copy Location" constraints of this empty.